Remove npm audit fallback request

[wraithgar]

When the new audit endpoint was added, in order for the cli to ship it without having to wait for a new semver major it was done with a fallback to the old endpoint:

https://github.com/npm/cli/blob/63d6a732c3c0e9c19fd4d147eaa5cc27c29b168d/workspaces/arborist/lib/audit-report.js#L332-L345

As far as we can tell, nobody has implemented an audit endpoint that relies on the old URL, and the new one works just fine on the npm registry.

This code should be removed in a semver-major release of npm.